1. Field
This disclosure relates to voice over packet networks, more particularly to providing full PBX feature transparency across voice over packet networks.
2. Background
If users move from a traditional, public switched transmission network (PSTN) phone system to a packet voice phone system, they can obtain several advantages. Voice over packet networks typically use existing data networks, avoiding large network change over costs. Coding and packetizing voice signals and routing them across existing packet networks also saves considerable costs on long-distance charges. Using excess capacity on existing data networks also makes those networks more efficient and cost effective.
Most of these advantages result from the connections to the outside world from a company's internal phone system. One example of an internal phone system is a PBX, or private branch exchange. Changing the traditional PBX to a packet voice network may cause some features to be lost.
In the ISDN (integrated services digital network) environment, there are two classes of service, basic and supplementary. The difference between the two classes lies in their ability to stand alone. Stand-alone services are basic services. Supplementary services rely upon the presence of another service to operate. Examples of supplementary services include call waiting, call hold, completion of call to busy subscriber, call transfer, call forwarding, etc. These will also be referred to as PBX features, since they are typically associated with PBX service. A packet voice network should maintain these features to provide users with no loss of features.
When packet networks replace PBXs, however, a problem can arise with certain types of control messages. Network devices in the packet network may drop control messages occurring between two directly connected nodes, referred to as local messages. Since the messages are only locally significant, the routers do not send them to other stations on the network. A problem arises when two nodes are connected through a third node. The middle node drops the locally significant messages, which may affect the network's ability to provide supplementary services. Some supplementary services depend upon the local messages to operate.
For example, when one end node disconnects from the network such as when the user hangs up, that node sends a disconnect message on the data (D) channel to the middle node. The middle node responds with a RELEASE message to the end node before passing the disconnect to the other end node. A RELEASE message releases the B channel used by the end node to connect with the middle node and ultimately the other end node. This sequence effectively prevents any supplementary services from being passed from the one end node to the other. Because the middle node does not pass on the RELEASE message, since it is of local significance, any supplementary services information in that message does not pass either. This is also true of any supplementary service elements being carried by the RELEASE COMPLETE messages.
Therefore, a method to provide supplementary services information contained in locally significant messages to non-local nodes on the packet voice network is needed. This would allow users to continue to have all the features available on traditional PBXs, with all the advantages of a packet voice network.